The invention relates to a method and a system for preventing oil escape into an exhaust gas during operation of a turbocharged engine.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other applications utilizing turbocharger units such as aero or marine systems.
A turbocharger unit is a vehicle component used together with an associated internal combustion engine, typically a diesel engine. The turbocharger configured to recover a part of the energy of the exhaust gas and to use that energy to compress intake air flowing into the combustion chamber of the internal combustion engine. Turbocharger units are commonly provided for increasing the efficiency and power of the internal combustion engine.
A turbocharger unit has three main components; a turbine for converting energy of the exhaust gas flow to a rotational movement of the turbine, a compressor rotationally connected to the turbine for compressing intake air, and a housing enclosing the turbine and the compressor as well as a rotating shaft, bearings, etc.
A turbine shaft, supporting the turbine, is supported in the bearing housing for allowing the turbine to rotate relative the housing. In order to minimize hot gas inflow from the gas path into the bearing housing supporting, the turbine shaft, as well as to prevent oil leakage in a reverse direction, sealing rings are used to seal between the static bearing housing and the rotating turbine shaft. As the static pressure level downstream the turbine wheel in some operating modes can be lower than the pressure inside the bearing housing a buffer air pressure is needed to prevent oil leakage.
US2013291828 describes seals being designed to help maintain a pressure differential between the relatively low pressure environment of the bearing housing and the potentially low pressure at the back face of the turbine in the turbine housing during operation of the engine in a normal fired mode. This prevents bearing oil (or other lubricant) leaking along the turbocharger shaft across the seals and into the turbine housing. Air is supplied to the turbine end seals so as to prevent leakage of oil into the turbine housing.
The same may be applied for a turbocompound unit arranged downstream of a turbocharger unit. A turbocompound unit is a vehicle component used for recovering a part of the energy of the exhaust gas flow and to convert that energy into a rotational movement of a shaft. The rotational movement of the shaft is transferred as a torque increase to the crankshaft of the engine of the vehicle. Normally, the turbocompound unit has a shaft wherein a turbine wheel is arranged at one distal end. When the internal combustion engine is running exhaust gas will flow into the turbocompound unit and cause the turbine wheel to rotate. Hence, the shaft of the turbocompound unit will rotate accordingly. The opposite end of the shaft is provided with a gear wheel which meshes with additional gears for causing a rotational connection between the shaft and the crankshaft. When the shaft is rotating due to exhaust gas flowing through the turbocompound unit, the rotational energy of the shaft will be transferred to the crankshaft as a torque increase.
The use of turbocompound units has proven to provide significant advantages on driving economics as well as on the environment the energy recovery from the exhaust gas flow will in fact reduce the fuel consumption of the vehicle.
The turbine shaft is supported in a bearing housing for allowing the turbine shaft to rotate relative the housing. In order to minimize hot gas inflow from the gas path into the bearing housing supporting the turbine shaft, as well as to prevent oil leakage in a reverse direction, sealing rings are, used to seal between the static bearing housing and the rotating turbine shaft. As the static pressure level downstream the turbine wheel in some operating modes can be lower than the pressure inside the bearing, housing a buffer air pressure is needed to prevent oil leakage.
Should the supply of air for some reason be subject to malfunction there is an immediate risk of leakage of crank case gases containing oil into the exhaust gas flow. Oil leakages into the exhaust gases will cause environment pollution or major failure on the exhaust after treatment system. Therefore such malfunction would cause an unplanned stop on road for the vehicle or otherwise lead to possible damages on the after treatment system.
It is desirable to provide a method and a system overcoming the above mentioned drawbacks of prior art units.
By detecting malfunction of the supply of pressurized fluid, and thereafter increasing the pressure inside the exhaust conduit downstream the turbine, a so called limp home mode is achieved whereby unplanned stops of the vehicle are avoided.
A method for preventing oil escape into an exhaust gas during operation of a turbocharged engine is therefore provided. The method comprises the steps of providing pressurized fluid to an area sealing off a bearing housing of an axial turbine unit from an adjacent exhaust conduit downstream of the axial turbine unit; and detecting a malfunction in said provision of pressurized fluid. In response to such malfunction detection, the method further comprises the step of controlling an exhaust pressure increasing device arranged downstream of the axial turbine unit for increasing the pressure inside the exhaust conduit upstream of the exhaust pressure increasing device.
In an embodiment, the step of detecting a malfunction is performed by measuring the pressure of said pressurized fluid, and by comparing said measured pressure value with a reference value. In another embodiment, the step of detecting a malfunction is performed by measuring the flow of said pressurized fluid, and by comparing said measured flow value with a reference value. Hence a pressure sensor or a tow sensor could be utilized, whereby a simple and robust construction is achieved.
In an embodiment the method further comprises the step of determining a desired operational status of the exhaust pressure increasing device. Following this, the exhaust pressure increasing device may be controlled to operate according to the desired operational status. Since the performance of the engine is affected by increasing the pressure inside the exhaust conduit upstream of the exhaust pressure increasing device, this is advantageous in that any degree of malfunction may be adequately compensated.
The step of determining the desired operational status may be performed by estimating a pressure difference between the bearing housing and the adjacent exhaust conduit, and calculating the desired operational status of the exhaust pressure increase device based on said estimated pressure difference. Over compensating the malfunction is thus avoided.
The pressure difference may be estimated based on the current engine torque and the current engine speed, which is advantageous in that dedicated pressure sensors are not required.
In an embodiment the step of estimating the pressure difference between the bearing housing and the adjacent exhaust conduit is performed by fetching a stored pressure difference value from a memory. As the pressure difference estimations are pre-stored, accessing such value is facilitated without the need for computational power.
The step of controlling said exhaust pressure increasing device may be performed by controlling the operation of a closed loop butterfly valve, or by controlling the operation of an exhaust pressure governor. Since these kinds of devices may be provided for allowing the engine to be operated in engine braking mode, no additional or tailor made components are required.
The step of controlling the exhaust pressure increasing device is in some embodiments performed to such an extent as to provide a positive pressure gradient from the turbo charger bearing housing, to the exhaust conduit, i.e. the pressure behind the turbine is higher than the pressure in the bearing housing. Oil leakage is thereby prevented, since the flow of gas is directed away from the exhaust conduit, into the bearing housing.
According to a further aspect, a computer program is provided comprising program code means for performing the steps of the method according to the first aspect when said program is run on a computer.
According to a yet further aspect a computer readable medium is provided, carrying a computer program comprising program code means for performing the steps of the method according to the first aspect when said program product is run on a computer.
According to a farther aspect a control unit for controlling an exhaust pressure increasing device is provided, the control unit being configured to perform the steps of the method according to the first aspect.
According to a yet further aspect a vehicle system is provided, comprising an exhaust pressure increasing device and an axial turbine unit arranged upstream of said exhaust pressure increasing device, said axial turbine unit comprising a fluid supply for providing pressurized fluid to an area sealing off a bearing housing of the axial turbine unit from an adjacent exhaust conduit. The vehicle system further comprises a control unit configured to monitor the operation of said fluid supply, wherein said control unit is further configured to control the operation of said exhaust pressure increasing device in response to a detected malfunction in said provision of pressurized fluid.
In an embodiment, the exhaust pressure increasing device is a closed loop butterfly valve or an exhaust pressure governor.
The vehicle system may further comprise a sensor for detecting the fluid flow and or the fluid pressure from said fluid supply.
In some embodiments the axial turbine unit is a turbocharger unit or a turbocompound unit.
The control unit may comprise a comparing unit configured to compare a measured value, representing the operation of the fluid supply, with a reference value. Further to this, the control unit may comprise a calculating unit configured to determine a value representing a desired operational status of the exhaust pressure increasing device, and wherein the control unit is further configured to control the exhaust pressure increasing device based on said determined value.
According to a yet further embodiment, a vehicle is provided comprising a vehicle system according to the aspect previously described.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.